The present invention relates to a thermal transfer recording method, such as sublimation type thermal transfer recording method.
There are known various thermal transfer recording methods. Of these methods, a sublimation type thermal transfer recording method is well known. In this method, sublimation dye as color material is transferred from an ink sheet to an image receiving sheet by means of a thermal head heated according to an input signal to obtain an image thereon. Recently, this method is utilized for recording an information in many fields. According to this recording method, since dyes of sublimation type are used as color materials, the gradation density of an image can be adjusted freely, and a full color image can be obtained on an image receiving sheet. Since an image formed by dyes is very clear and has a good transparency, a high quality image with excellent reproducibility of half tone and gradation density equal to a silver salt photographic image can be recorded thereon.
As one of image receiving sheets used in the sublimation type thermal transfer recording method, a sheet which comprises a substrate formed of normal paper is proposed. A printed matter formed by printing on the sheet having the paper substrate described above has good characteristics as follows; (1) a brightness or feeling of the surface of a printed matter is nearly equal to that of a matter having an image which is formed through a normal printing method; (2) the image receiving sheet having the paper substrate can be bent while a conventional image receiving sheet having a synthetic paper substrate cannot be bent; (3) a printed matter can be book-bound and filed in a state where several sheets are piled. These characteristics make possible wide uses. In addition, the image receiving sheet can be produced at a reasonable cost since a normal paper for the image receiving sheet is cheaper than the synthetic paper substrate thereof.
However, a cushioning property of a paper and smoothness of the surface thereof are small, a high quality image and a high printing sensitivity cannot be obtained from the image receiving sheet having only a color receptor layer on a paper substrate. As the related arts of the above mentioned techniques, Japanese Laid-Open Publication No. 270147/1993 and No. 270152/1993 disclose an image receiving sheet in which a foamed layer comprising foam agent and resin is formed on a substrate to complement a cushion or a cushioning property and a heat insulation property of the substrate.
In the image receiving sheet with the foamed layer, an unevenness due to bubbles occurs on the surface of the color receptor layer to obstruct a tight contact between a thermal head and an image receiving sheet, a printing density and an image quality are decreased. In order to solve the problem thereof, the inventors proposed a method of smoothing the surface of the foamed layer in which a metallic roll with a specular finish is contacted with the surface of the foamed layer after forming the foamed layer on the paper substrate as disclosed in Japanese Laid-Open Publication No. 210968/1994. However, the above-mentioned method requires complicated production processes. In the above-mentioned method, although there is a secondary advantage that an unevenness of the surface of an image receiving sheet due to bubbles in the foamed layer can give a natural mat-like feeling to the surface of the image receiving sheet and a normal paper-like feeling, the advantage is lost by smoothing treatment on the surface of the foamed layer.
The inventors filed a patent application for an invention of an image receiving sheet in which a high quality printing image may be formed thereon by formation of a soft intermediate resin layer on the foamed layer even if an unevenness due to bubbles occurs on the surface of the color receptor layer, and in which the bubbles in the foamed layer are protected against a thermal shock and a mechanical shock by the thermal head.
However, even if the image receiving sheet having the soft intermediate resin layer on the foamed layer is used, there is easily caused degradation of a quality of a printed image formed at a low environment temperature or under an insufficient heating condition in a printer. As a concrete example of the above-mentioned problem, there is a case that a dropout in a printed area or a rough feeling on the surface is caused in a region having a low image density, so that letters or fine lines are broken off and become blurred. This tendency is remarkable in a single color image.
Although the cause of the above-mentioned phenomenons is not clear, it is presumed as follows; the composition material for a color receptor layer, an intermediate layer and a foamed layer in the image receiving sheet described above mainly comprises a polymer resin and an inorganic or organic addition agent of many kinds added to the polymer resin described above. Therefore, at a low temperature, a softness of the composition material with a resin is decreased to make worse a contact condition between the thermal head and the image receiving sheet. Accordingly, the dye cannot be transferred to a concave part of an uneven surface of the image receiving sheet to deteriorate an image quality.
The object of the present invention is to provide a thermal transfer recording method in which a high quality image can be obtained using the above-mentioned image receiving sheet even in the case of a low environment temperature.